[Morphological and anatomical data on several cases of duplication in Nereidae: Morphogenetical interpretation].

The authors describe 12 cases of duplication (6 without bifurcation of the body et 6 with bifurcation of the body) in the caudal part of three species of Nereids (Nereis pelagica L.,Nereis diversicolor O.F.M.,Perinereis cultrifera G.)Duplication without bifurcation of the body concerns the dorsal blood vessel alone (1 case) or segmentary structures i.e. parapodia, nerve cord, nephridia and longitudinal ventral muscles (5 cases). The bifurcation of the body arises either in the forntal plane (1 case) without alteration of segmentary organogenesis of the supernumerary tail, or in the sagittal plane on the dorsal (4 cases) or ventral (1 case) face with modification of the segmentary organogenesis; the dorsal bifurcation have no parapodia, nerve cord, nephridia and longitudinal ventral muscles while the ventral bifurcation possesses twice this lot of structures.The comparative morphological and anatomical study of these abnormal Annelids leads us to consider:The bifurcation of the body as resulting from only junction between ventral or dorsal body wall and the intestine;The alterations of segmentary organogenesis as a consequence either of the duplication or the absence of the nerve cord, the number of pairs of structures like parapodia, nephridia and longitudinal ventral muscles corresponding to the number of nerve cords present in the segment.